Experimental study on flame height and radiation characteristics in jet flames with ammonia/hydrocarbon mixture fuels

Abstract

Ammonia is considered as a renewable fuel and expected to play an essential role in coping with the energy crisis and climate change. Because of low combustion rate and high NO emission of ammonia, many studies focused on the characteristics of ammonia/hydrocarbon mixture fuels. However, the flame height and radiation characteristics of jet flames of ammonia and hydrocarbon mixture fuels have not been systematically studied yet. In this work, hydrocarbon fuels of methane and ethylene are mixed with ammonia for the fuel mixing ratio in the range of 0 to 50 %. Results show that the flame height increases with heat release rate (HRR) but changes little with the concentration of ammonia. The previous physical models for turbulent jet flames based on an integral approach can basically predict the flame height for heat release rate larger than 2.7 kW and a semi empirical formula based on dimensional analysis is verified well in predicting the flame height with the flame Froude number for Fr_f > 0.05. For laminar flame heights, a model is proposed here considering the buoyancy effect which predicts well for CH₄/NH₃ flame with total flow rate smaller than 5 L/min. For thermal radiation characteristics, the radiation fraction decreases with the ammonia mixing ratio increases. A consistent transitional value in predicting the radiation fraction is observed, irrespective of fuel type, which is mainly due to the reduction of soot during the transition from laminar to turbulent flames. The influence of ammonia on the flame radiation fraction of CH₄/NH₃ flames normalized by that of pure hydrocarbon fuel (${\chi }_R/{\chi }_{R_0}$) can be correlated by the mixing ratio. For C₂H₄/NH₃ flames, the influence of ammonia on the dimensionless radiation fraction is the result of the coupling effect of mixing fraction and strain rate. A correlation is proposed considering the effects of ammonia addition on radiation fraction in relation to Reynolds number, which is shown to correlate well the experimental results.

Novelty and significance statement

As ammonia/hydrocarbon fuel mixtures can reduce the emission of carbon dioxide, it can play an essential role during the transition to a non-carbonization society. Considering a fundamental importance of jet flames and a fire hazard scenario in case of fuel leakage, the present study investigates the effect of ammonia addition on the flame height and flame radiation fraction for ammonia/hydrocarbon fuel mixtures. We test the physical model of Quintiere and a semi empirical formula of Delichatsios in predicting the turbulent flame height of ammonia/hydrocarbon mixture fuels. A normalized model is proposed to describe the effect of adding ammonia on radiation fraction in relation to the Reynolds number. Our results provide necessary experimental data and models for further study on ammonia/hydrocarbon mixture fuel flames.